1. Field of the Invention
The present invention relates to the vapor deposition of silicon-containing amorphous thin films, and in particular to vapor deposition of silicon-containing non-polymeric amorphous thin films with significantly reduced optical absorption (reduced optical loss) in the near-infrared optical communications wavelength region of about 1.45 to 1.65 microns for such uses as in the fabrication of optical devices, such as optical waveguides, ring resonators, arrayed waveguide grating multiplexers/demultiplexers, optical add/drop multiplexers, optical switches, variable attenuators, and dispersion compensators.
2. Background of the Technology
Conventional vapor deposition methods used for growing silicon-containing amorphous thin films on substrates for optical device applications typically rely on at least one hydrogen-bearing source gas. For example, silicon-oxide (SiO2) 1 or silicon-oxynitride (SiOxNy) 2 thin films, as shown in FIG. 1, are grown on a substrate 3 using low pressure chemical vapor deposition (LPCVD) (see, e.g., J. Yota, J. Hander, and A. A. Saleh, xe2x80x9cA comparative study on inductively-coupled plasma high-density plasma, plasma-enhanced, and low pressure chemical vapor deposition silicon nitride films,xe2x80x9d J. Vac. Sci. Technol. A 18 (2), 372 (2000)) or plasma enhanced chemical vapor deposition (PECVD) (see, e.g., L. Martinu and D. Poitras, xe2x80x9cPlasma deposition of optical films and coatings: A review,xe2x80x9d J. Vac. Sci. Technol. A 18 (6), 2619 (2000)). These techniques generally rely on silane (SiH4) and ammonia (NH3) as source gases for silicon and nitrogen, in combination with an oxygen bearing gas, such as O2 or N2O. The resulting as grown SiOxNy films contain substantial amounts of hydrogen (2-25%) in the form of Sixe2x80x94H, Nxe2x80x94H, and Oxe2x80x94H bonds. The presence of atomic hydrogen affects many of the film""s physical properties, including density, porosity, optical absorption, index of refraction, hardness, and stress.
One problem with near-infrared optical device applications using these materials is optical absorption, or loss, in the near-infrared optical communication wavelength region from 1.45 to 1.65 microns. The absorption occurs at least partially due to an effect commonly referred to as xe2x80x9cstretching modexe2x80x9dxe2x80x94the motion of atoms that occurs in perpendicular directions on the same axis, away from each other. As a result, simple optical waveguides having a waveguide core consisting of vapor deposited SiOxNy show optical losses of 10 dB/cm and higher for optical wavelengths near 1.51 microns, which results from optical absorption by the overtones of the vibrational stretching modes of Sixe2x80x94H and Nxe2x80x94H bonds (see, e.g., G. Grand, J. P. Jadot, H. Denis, S. Valette, A. Fournier, and A. M. Grouillet, xe2x80x9cLow-loss PECVD silica channel waveguides for optical communications,xe2x80x9d Electronics Letters 26 (25), 2135 (1990)). In addition, there is also significant infrared absorption near 1.4 microns, resulting from the presence of Oxe2x80x94H bonds.
One way to eliminate this effect is to remove the hydrogen from the substance through which the light is being transmitted. A technique commonly used to accomplish this removal is to anneal the films at high temperatures (xcx9c1140xc2x0 C.), driving as much of the hydrogen from the film as possible (see, e.g., R. Germann, H. W. M. Salemink, R. Beyeler, G. L. Bona, F. Horst, I Massarek, and B. J. Offrein, xe2x80x9cSilicon oxynitride layers for optical waveguide applications,xe2x80x9d Journal of the Electrochemical Society 147 (6), 2237 (2000)). This technique can substantially reduce the optical loss in the wavelength region of interest to below 1 dB/cm, but at the expense of an additional process step that can cause shrinkage of the film and introduce significant tensile stress in the film. These effects can create cracks in the film and bowing of the wafers.
These effects occur because the annealing temperature is high enough to drive hydrogen atoms out of the film, but not high enough to melt the film and allow it to flow and reshape. The resulting stretch or bending of the wafer makes the wafer difficult to process using lithography and standard semiconductor processing techniques. Finally, this process results in a number of dangling bonds of silicon atoms remaining in the film, and if the film is later exposed to sources of hydrogen, such as water vapor from humidity, the hydrogen can react and reattach to the silicon, eventually resulting in the same problem with absorption that was present absent annealing.
Low optical losses at near-infrared wavelengths have been achieved in organic polymer devices and optical fibers by making use of deuterated and halogenated materials, but these methods and devices are not useful for integrated and other non-polymeric optical devices. (See, e.g., U.S. Pat. No. 5,062,680 to S. Imamura; U.S. Pat. No. 5,672,672 to M. Amano; T. Watanabe, N. Ooba, S. Hayashida, T. Hurihara, and S. Imamura xe2x80x9cPolymeric optical waveguide circuits formed using silicone resin,xe2x80x9d Journal of Lightwave Technology 16 (6), 1049 (1998); U.S. Pat. No. 6,233,381 to Borrelli et al.) Deuterated gases have also been applied to the field of semiconductor electronics to create insulating and passivation layers in semiconductor transistor devices for such purposes as to mitigate hot-electron effects in gate oxides, but these methods and devices have no applicability to integrated and other non-organic optical devices, nor are the purposes for which deuterium is used in semiconductor transistor devices generally useful for producing optical devices. (See, e.g., U.S. Pat. No. 5,972,765 to W. F. Clark; U.S. Pat. No. 6,025,280 to D. C. Brady; U.S. Pat. No. 6,023,093 to Gregor et al.; U.S. Pat. No. 6,077,791 to M. A. DeTar.) Each of the references referred to herein is hereby incorporated by reference in its entirety.
There remains an unmet need to provide optical devices, including non-polymeric passive optical devices and integrated optical devices that have low optical losses at selected wavelengths. There is a further need to provide devices and methods of making devices for use with waveguides on wafers, such as planar lightwave circuits, including circuits with multiple devices connected by waveguides on a single wafer, that incorporate other processes than annealing and overcome the problems with this technique.
The present invention relates to optical devices, including integrated optical devices, and methods for fabrication via vapor deposition of non-polymeric, silicon-containing thin films using vapor sources, such as deuterated liquids, comprised of deuterated species. With embodiments of the present invention, thin films are grown on a substrate to form optical devices or portions thereof that have at least one deuterium containing layer. These devices have significantly reduced optical absorption or loss in the near-infrared optical spectrum, which is the spectrum commonly used for optical communications, compared to the loss in waveguides formed in thin films grown using conventional vapor deposition techniques and hydrogen containing precursors.
The devices produced in accordance with embodiments of the present invention have deuterium in place of hydrogen within bonds for the formed films. Deuterium, which is an isotope of hydrogen that has a neutron in its nucleus, vibrates within bonds with other atoms at frequencies different from hydrogen in the same bonds. This difference in frequency results from the increased mass of deuterium over hydrogen. Because of the different frequency of vibration of the deuterium in these bonds, relative to hydrogen, different wavelengths of energy, including light, are absorbed within the materials formed using deuterium than the wavelengths absorbed by the same materials when hydrogen is present. Deuterium used in the formation of optical devices in accordance with the present invention results in shifts of energy peaks for these materials, such that the primary band of wavelengths to be transmitted, in the 1.45 to 1.65 micron range, are no longer absorbed by the material of these devices.
In one embodiment of the present invention, deuterated gases (gases and vapors are used interchangeably herein), such as SiD4 and ND3 (D being deuterium), serving as precursors, along with a gaseous source of oxygen, such as nitrous-oxide (N2O) or oxygen (O2), are used for the chemical vapor deposition of silicon-oxynitride (SiOxNy:D) or other non-polymeric thin films on a cladding. The cladding is composed, for example, of silicon oxide (SiO2), phosphosilicate glass, fluorinated silicon oxide, or SiOxNy:D having an index of refraction less than that of the thin film. In an embodiment of the present invention, the cladding is formed on a substrate, such as silicon, quartz, glass, or other material containing germanium, fused silica, quartz, glass, sapphire, SiC, GaAs, InP, or silicon. In embodiments of the present invention, the thin film and the cladding formed on the substrate can vary in thickness and width, depending, for example, on the device being formed. In embodiments of the present invention, the cladding is formed with a thickness varying from 2 to 20 microns, and the thin film is formed with a thickness varying from about 0.5 to 5 microns. Other thicknesses of the cladding and the thin film are also usable in accordance with the present invention.
In accordance with embodiments of the present invention, ridge structures can be formed from the thin deuterium containing films such as SiOxNy, Si3N4, or SiO2, by an etching process, such as reactive ion etching (RIE), to form an optical waveguide, one basic building block of integrated optical devices.
Embodiments of the present invention include formation of the integrated or other optical devices on substrates that include or have formed upon them other electronic or optical devices, or formed portions thereof, referred to herein as xe2x80x9cpreformed devices.xe2x80x9d These preformed devices can include, for example, field-effect transistors (FETs), such as metal-oxide-semiconductor FETs (MOSFETs), electronic amplifiers, preamplifiers, devices containing pn junctions, transformers, capacitors, diodes, laser drivers, lasers, optical amplifiers, optical detectors, optical waveguides, modulators, optical switches, or other electronic or optical devices. These examples are intended to be merely illustrative of integrated and other devices upon which the thin film of the present invention may be formed. The present invention has the advantage over the prior art that formation on these devices is possible because no annealing is required, which, in the prior art, potentially damages the devices on which the film is grown or otherwise formed.
In other embodiments of the present invention, rather than a silicon-oxynitride film, silicon nitride (Si3N4) or silicon-oxide (SiO2) films are grown using the techniques of the present invention to eliminate either the oxygen bearing gas or the deuterated ammonia gas, respectively. As with films using silicon-oxynitride, by using deuterated gases instead of the hydrogenated versions of silane and ammonia (SiH4 and NH3), the resulting thin films have virtually zero hydrogen content and instead contain some deuterium.
Examples of deuterated liquids usable to produce the deuterated gases include deuterated tetraethoxysilane, deuterated tetraethylorthosilicate, deuterated hexamethyldisiloxane, deuterated hexamethyldisilazane, deuterated tetramethoxysilane, and deuterated tetramethyldisiloxane. Examples of precursors containing deuterium include SiD4, Si2D6, SiDCl3, SiCl2D2, ND3, GeD4, PD3, AsD3, CD4, and D2S.
Layers may be formed on substrates using deuterated gases, in accordance with embodiments of the present invention, via any of a number of chemical vapor deposition and other techniques known in the art for forming thin layers on integrated components, including plasma enhanced chemical vapor deposition (PECVD), high density plasma chemical vapor deposition (HDPCVD), low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), jet vapor deposition (JVD), flame hydrolysis, and electron cyclotron resonance (ECR) chemical vapor deposition. Embodiments of the present invention include replacing hydrogen atoms and/or molecules in the source gas species with deuterium in order to virtually eliminate the population of hydrogen in the growth chamber, and the resulting films grown by this method demonstrate complete replacement of incorporated hydrogen with deuterium atoms.
Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the following or upon learning by practice of the invention.